1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for allocating frequencies among cell clusters, and specifically to alternating the antenna pointing azimuth angle between rows of cell clusters to minimize co-channel interference.
2. Background and Objects of the Present Invention
Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications ever. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world. Cellular networks have evolved into two different networks. The European cellular network uses the Global System for Mobile Communication (GSM) digital mobile cellular radio system. In the United States, cellular networks are still primarily analog, but many North American cellular networks have begun deploying Signaling System #7 (SS7), which is a digital signaling standard, to support access of remote databases. European cellular networks have always relied on SS7 for their signaling requirements. However, GSM is currently operated in North America in a newly reserved frequency band in the 1900 MHZ range. The revised GSM standard is also known as Personal Communication Services 1900 or PCS 1900. FIG. 1 illustrates the typical components of a GSM/PCS 1900 wireless communications system, generally designated by the reference numeral 10.
The GSM/PCS 1900 wireless communications system 10 is located within a geographical area serviced by a single provider, hereinafter also referred to as the Public Land Mobile Network (PLMN) 10. The basic components of the wireless communications system 10 are a Base Station System (BSS) 25, a Mobile Switching Center (MSC) 14 and connected Visitor Location Register (VLR) 16 and a Mobile Station (MS) 20. At least one BSS 25 is deployed within the PLMN 10. The BSS 25 acts as an interface between the MSC 14 and a number of MSs 20, each of which may be a mobile wireless telephone, a pager or other equipment.
The MSC/VLR areas 12 include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a mobile station (MS) 20 may move freely without having to send update location information to the MSC/VLR area 12 that controls the LA 18. Each Location Area 18, in turn, is divided into a number of cells 22, only one of which is shown in FIG. 1.
The BSS 25 includes a Base Transceiver Station (BTS) 24 and a Base Station Controller (BSC) 23. At least one BTS 24 operates as a transceiver for transmitting and receiving data and control messages to and from the MS 20 over the air interface within the cell 22. The BSS 25 is generally connected to the MSC 14 through dedicated telephone lines through an A-interface 15. Also connected to the MSC 14 is a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 2.
The VLR 16 is a database containing information about all of the MSs 20 currently located within the MSC/VLR area 12. If a given MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that Mobile Station (terminal) 20 from the HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
Each service provider has a specified number of frequencies which can be used within the PLMN service area 10 assigned to the service provider. These frequencies are divided up among each of the cells 22. Due to the large number of cells 22 and small number of allowed frequencies, frequency re-use patterns are typically used by service providers to support cellular service for all customers within the PLMN service area 10.
As shown in FIG. 2 of the drawings, frequency re-use patterns are cell-based structures 240, designated therein by the bold outlines, by which the frequency channels within a cellular system 205 are assigned. The most basic unit of any frequency re-use pattern is the cell, designated in FIG. 2 by the reference numeral 200. Each cell 200 in a frequency re-use pattern is assigned a number of frequency channels. A group of cells 200 associated together are also referred to as a cluster 240, which preferably contains all of the frequency channels available to a particular cellular system 205. Groups of clusters 240 are then used to provide cellular coverage over a specific area of the cellular system 205. The association of all frequency channels within a single cluster 240 enables the re-use of the frequency channels throughout the cellular system 205.
The particular cell planning structure illustrated in FIG. 2 is a center-excited, sectorized, seven-cell cluster 240, in which each cluster 240 has seven cells 200, each of which are further divided into three sectors 210, 220 and 230. Each cell 200 within each cluster 240 is assigned a particular frequency group, which is divided into three sub-frequency groups for each of the three sectors 210, 220 and 230, respectively.
It should be understood that each cell 200 contains six antennas (not shown) located at the center of the respective cells 200, two for each sector 210, 220, and 230 therein. One antenna in each sector 210, 220, and 230 transmits messages to the mobile terminals 24 within the respective sectors 210, 220 and 230, while the other antenna in each sector 210, 220 and 230 receives messages from the mobile terminals 24 within the respective sectors 210, 220 and 230. It should be understood that both the receiving and transmitting antennas in one respective sector 210 point along the same direction, angularly displaced from the other antenna pairs in the other respective sectors 220 and 230 in the respective cell 200 by 120 degrees.
For example, in FIG. 2 the uppermost sector 210 in each cell 200 has an antennae pair pointing 30 degrees from north, as shown by the arrow. This pattern is repeated for each lowermost sector 220, which would therefore have its antennas (again represented by an arrow) pointing 150 degrees from north, and each sidemost sector 230, which would therefore have its antennas pointing 270 degrees from north. The degrees with reference to north are hereinafter referred to as antenna-pointing azimuths 215, represented in FIG. 2 by the arrows. Thus, the cells 200 in FIG. 2 have antenna-pointing azimuths 215 of 30 degrees.
As shown in FIG. 2, the clusters 240 in the cellular system 205 are structured and the frequencies within the clusters 240 are allocated therein to increase the re-use distance and to limit co-channel and adjacent channel interferences. Co-channel interference is the interference caused by the usage of the same frequency within two different cell clusters 240. Adjacent channel interference is caused by interference between adjacent cells 200 and frequency channels within the same cluster 240 or within two different clusters 240. In order to reduce interference within the cellular system 205, both co-channel and adjacent channel interference must be minimized. Competing with these requirements is the need for increased system capacity. In general, the smaller the number of cells 200 used in a cluster 240 within a cellular system 205, the higher the capacity of the system and the lower the co-channel re-use distance. A smaller co-channel re-use distance, of course, increases co-channel interference.
The co-channel interference ratio, e.g., Carrier/Interference (C/I), is very critical to improving the network performance in cellular networks 205, including, but not limited to the GSM network, the new Personal Communications System (PCS) network, the D-AMPS network, and the AMPS network. Therefore, by increasing the ratio, e.g., by reducing the interference with respect to the carrier (level) of the desired signal, the co-channel interference is reduced and the signal quality received by mobile terminals 24 within the cell 200 is improved.
It is, therefore, an object of the present invention to reduce the co-channel interference between cell clusters, thereby improving signal quality.